The production of chlorine by electrolysis of alkali halide solutions, in particular of sodium chloride solutions, is still largely the electrochemical process of higher industrial relevance. It may be carried out by resorting to the three technologies of membrane electrolysis, diaphragm electrolysis and mercury cathode electrolysis.
The subsequent evolution of diaphragm plants has led to the introduction of polymer diaphragms made hydrophilic by means of various additives, for instance, zirconium oxide fibres or particles, instead of the traditional asbestos ones, overcoming the main inconvenience of this technology from an environmental standpoint. From the point of view of energy consumption, several improvements have been introduced which have slowed down the disposal of diaphragm cells in favour of the membrane technology, which initially appeared to be unavoidable. At first, the conventional expandable anodes of titanium activated with noble metal oxides were substantially improved by means of a so called zero-gap version, that is, provided with devices capable of exerting an elastic pressure and of bringing the movable surface of the anode into a direct and extended contact with the diaphragm. These anodes were later provided with double expanders, such term indicating the connections which allow the passage of electric current from the anode movable surfaces to the current collecting stems, with a sensible reduction of the relevant ohmic drop. Moreover, devices allowing to sensibly increase the brine internal recirculation may be advantageously installed on the anodes, with a consequent advantage in terms of lower voltage and lower oxygen release, both of which elements allow decreasing the energy consumption per ton of produced chlorine.
Finally, the replacement of the rubber linings with titanium sheets to protect the copper bases whereon the anodes are fixed and the adoption of new kinds of elastic gaskets between cathode body and anode support base and between each anode and the support base allowed to remarkably extend the operative life-time of the individual cells making up an electrolysis plant, from which followed a further decrease of the maintenance costs and a higher productive capacity for a given cell design.
A description of the functioning of chlorine-caustic soda diaphragm cells is provided in a very clear fashion in Ullmann's Encyclopaedia of Chemical Technology, 5a Ed., Vol. A6, pg. 424-437, VCH, while details of the internal structure of such cells are exhaustively illustrated in the drawings of the prior art. Only after several modifications made to the diaphragms and the anodes with the relevant way of fastening to the support base, the attention was recently focused on the cathodes, indicating with this term both the body with the relevant electrical connections and the structure of the cathode active area which is the site of the hydrogen release reaction and of the generation of caustic soda. In particular, as regards the cathode active area, this consists of a mesh of interwoven wires or of a perforated sheet both made of conductive material, generally carbon steel, shaped so as to form structures similar to prisms with a rather flattened rectangular section, secured by welding to a perimetrical chamber, also made of interwoven wires or perforated sheets, connected to the side-walls of the cathode body and provided with at least one nozzle in the lower part for the outlet of the solution containing the caustic soda product and the residual sodium chlorine, and with at least one nozzle in the upper part for the discharge of hydrogen. On these structures, known among those skilled in the art as fingers (a wording which will be therefore adopted in the following), the diaphragm is deposited by vacuum sucking of an aqueous suspension containing polymer fibres and particles which constitute the diaphragm itself, as mentioned above. In the diaphragm cell structure, the diaphragm-covered fingers are intercalated with the anodes, whose surface can be either in contact with that of the diaphragms or be spaced by a few millimetres. In both cases, it is necessary that the fingers do not undergo deflections which would cause abrasions on the diaphragm with consequent damaging thereof. Furthermore, during operation the current must be transmitted in the most uniform possible fashion to the whole finger surface. A non-uniform distribution would cause a cell voltage increase and a reduction of the caustic soda generation efficiency with a simultaneous higher oxygen content in the chlorine. It follows that for a better result, the fingers must be provided with adequate stiffness and at the same time with high electrical conductivity.
According to the prior art, the fingers are provided with a longitudinally corrugated carbon steel or copper internal plate. The mesh of interwoven wires or perforated sheet is secured, preferably by welding, to the apexes of the corrugations, solving the problem of homogeneous current distribution and of stiffening.
Nevertheless, the corrugations developed in a longitudinal direction do not allow the hydrogen bubbles to rise freely in the vertical direction, to gather along the finger upper generatrix and then to penetrate into the perimetrical chamber provided as mentioned with at least one gas discharge nozzle. The longitudinally corrugated plate forces hydrogen to gather under each of the corrugations and to flow longitudinally along each of the corrugations until discharging across suitable perforations to the perimetrical chamber. Since this flow can hardly be equalised, it follows that the amount of hydrogen present under each corrugation is variable and occludes the facing diaphragm fraction to a different extent. In a last analysis, it can be said that the longitudinally corrugated internal plate determines an inevitable unbalance in the electric current distribution. Such an unbalance, in its turn, leads to a differentiation of caustic concentration, with a negative outcome on the faradic production yield and on the oxygen content in the chlorine.
The prior art also discloses the use of corrugated internal plates, but with vertically arranged corrugations. In this case it is apparent that hydrogen can freely gather in the finger upper part. However, its flow toward the perimetrical chamber is hindered by the upper part of the corrugations. Moreover, for a given electric current distribution, the stiffening effect of the vertical corrugations may be unsatisfactory.
There has also been disclosed designs for the finger internal element in which perforated horizontal plate strips or longitudinal conductive stems are provided with vertical plate strips welded thereto. While certainly assuring an adequate stiffness, the latter solution is affected by the problem of difficult hydrogen discharge previously discussed.
While some prior art designs represent a satisfactory answer to the requirements of stiffness, current distribution homogeneity and free hydrogen discharge, they have done so at the cost of a complex structure, difficult to produce and hence excessively expensive. Furthermore, the structures do not allow the hydrogen bubble upward motion to establish an adequate recirculation of the caustic soda product inside the fingers. As a consequence of this lack of recirculation, pockets of caustic soda of higher concentration may be formed, particularly in the case of anomalies in the electric current distribution and in the porosity of the diaphragms, with negative consequences as regards the electrolysis faradic efficiency and the oxygen content in the chlorine.
A more advanced solution, overcoming to a great extent the above illustrated drawbacks of the cathode finger internal plates, has been proposed in which the plate inside the cathode fingers is provided with bumps, for instance spherical caps, arranged quincuncially, or in accordance with other patterns which facilitate the free circulation of the electrolyte while providing for an electrical connection of well distributed ohmic paths.
There has been described the use of this plate for fingers obtained both out of meshes of interwoven wires, and out of perforated sheets. As a matter of fact, if in the latter case the proposed solution appears to be entirely optimal, in the former case the coupling between mesh and bump plate is in many cases unsatisfactory. Not every wire of the mesh can in fact intercept the various protrusions correctly, and statistically many of them cannot transport the current in an effective way, since the relevant ohmic path results excessively long.
It is thus identified the specific need to find an improved design of internal reinforcing and current-distributing structure for diaphragm cell cathode fingers consisting of conductive meshes, for example interwoven wire meshes.